1. Field of the Invention
The present invention relates generally to the synthesis of digital halftone images and relates specifically to a method and to an apparatus enabling color or black/white bi-level or multi-level halftone images to be displayed or printed on various raster output devices, in particular on display devices (cathode ray tubes, plasma displays, liquid crystal displays) or on printing devices (printers, telefaxes).
The method used to display color images can be reduced to a case of black/white image display if the color image is considered to be separated into three independent color planes (red-green-blue or cyan-magenta-yellow) or into four independent color planes (cyan-magenta-yellow-black), each independent color plane being treated as a single halftone black/white image. Therefore, the present description mainly refers to halftone black/white images, the color extension being implied.
2. Description of the Prior Art
In the printing industry, one of the common methods for reproducing halftone images using bilevel printing devices is the ordered dither method. This method consists in subdividing the whole output image space into repetitive adjoining rectangular areas-screen elements. The inside of each screen element is gradually blackened according to the gray level of the original image, thus ensuring the presence of various gray levels in the reproduction. According to the method used to fill in the screen elements, it is possible to distinguish two main families of dithering methods: a) dispersed-dot ordered dithering. and b) clustered-dot ordered dithering.
Dispersed-dot dithering was introduced by Bayer in 1973. Judice, Jarvis and Ninke proposed a simple recurrent method to compute a well-dispersed dither matrix. In the case of dispersed-dot dithering, the black dots are distributed within each dither element so as to ensure maximal dispersion. This method produces relatively good results when dealing with fine details (for instance, small objects, letters). Moreover, the number of distinct intensity levels which can be obtained using this method is not limited since the screen elements can be as large as necessary. However, a visually disturbing horizontal and vertical ruling may appear on the image produced using the dispersed-dot method. FIG. 2 shows the result of dispersed-dot dithering halftoning with its inherent artifacts. FIG. 4 shows 256 different patterns corresponding to the 256 distinct intensity levels which are reproduced using conventional dispersed-dot ordered dithering of size 16.times.16. A strong visually disturbing structure is noticeable at several intensity levels. Another big disadvantage of dispersed-dot ordered dithering is its very non-linear reproduction function which is clearly visible on the vertical gray ramp in FIG. 2.
In the case of clustered-dot dithering, which is the method most commonly used for high and medium resolution devices (.gtoreq.200 dpi), the black dots are clustered in the middle of each screen element, thus forming round figures. The images produced using this method are quite faithful to the original and are visually pleasing. However, in the case of a medium resolution device (200-600 dpi), the number of distinct gray levels which can be obtained using this method is greatly restricted, due to the discrete nature of the grids. FIG. 1 shows the result of clustered-dot ordered dithering halftoning. A clear banding phenomenon is visible on a vertical gray ramp; small details are poorly rendered. FIG. 3 shows 33 different patterns corresponding to the 33 distinct gray levels produced with conventional clustered-dot ordered dithering, when elementary screen element area equals 32 pixels. This halftone pattern is currently used in 300 dpi printing devices. Holladay has developed a method for defining dither matrices for clustered-dot dither (U.S. Pat. Nos. 4,149,149 and 4,185,304). This approach is limited to "rational tangent screening", that is to the screening technique where the orientation of screen cells is defined by a ratio of integer vertical and horizontal displacements between cells. An improvement of rational tangent screening can be obtained by making use of "supercell" techniques. Schiller (European Patent Nos 0,427,380 A2 and 0.498,106 A2) and Schiller and Knuth (European Patent No. 0.499,738 A2) proposed several implementations which make use of multi-cell threshold arrays. Other methods of using multiple threshold matrices to generate halftone images, were proposed by Kawamura (U.S. Pat. No. 4,752,822) and by Troxel (U.S. Pat. No. 5,124,803). In general, in "supercell" techniques, all elementary screens cells which form a supercell have identical form, but threshold values of corresponding elements vary from one cell to another.
The invented process which is described in the present disclosure is an improvement of the dispersed-dot dithering. Using the disclosed method, we obtain a method where the black dots are clustered inside the figures in the shape of rings and diagonally oriented crosses (see FIGS. 5 and 6). Thus, the invented process offers the advantages of dispersed-dot dithering while minimizing its disadvantages. The invented process has turned out to be particularly effective for 300 and 400 dpi ink-jet printers as well as for 300, 400, 600 and 800 dpi laser printers.
The invented method differs considerably from the "supercell" technique. A rotating threshold matrix obtained using the disclosed method contains smaller sub-matrices which, after rotation, vary in arrangement from sub-matrix to sub-matrix. Nevertheless, the set of threshold values remains the same for all sub-matrices which form the rotating threshold matrix.